1. Field of the Invention
The present invention relates generally to cordless telephony and, more particularly, to a method for avoiding interference among multiple communications systems.
2. Background of the Invention
The ISM (Industrial Scientific Medical) band includes 902-928 MHz, 2.4-2.483 GHz, and 5.725-5.875 GHz frequency ranges. In the United States, the ISM band is unlicensed, which means that any of the ISM band frequency ranges can be used in a variety of applications as long as Federal (FCC) Communications Commission's part 15 rules are followed. One such application involve cordless telephones, which operates in each of the 900 MHz, 2.4 GHz, and 5.8 GHz ranges. Another application of the ISM band involves Bluetooth (BT) wireless technology, which uses the 2.4 GHz range frequencies.
BT technology is used by in cellular telephones to enable communications between a cordless headset (e.g., one that can be worn on a user's head to enable “hands-free” operations) and a cellular telephone (e.g., one that can be attached to the user's belt). In such application, a BT transceiver is housed within each of the cordless headset and the cellular telephone. Since the BT transceivers and the cellular telephone network operate at different frequency bands, no significant interference is experienced.
Similar utility of BT technology has not been made in cordless telephones. This is because incorporation of BT technology in the cordless telephone sector would likely cause an unacceptable interference in the 2.4 GHz ISM band. For example, in a situation in which a BT transceiver is located in a cordless handset that already has a conventional 2.4 GHz cordless telephone transceiver, significant interference can be expected. Due to the significant interference, an acceptable communications session between a BT headset and the cordless handset may not be achievable. Currently, there is no known solution that would enable a cordless telephone to communicate with a BT headset.
Multiple access systems are well known in art. Signals that occupy non-overlapping frequency bands can be easily separated using appropriate band pass filters. Therefore, signals from two transceivers can be simultaneously transmitted without interfering each other as presented on FIG. 1. In this method, multiple access capabilities is achieved in frequency domain and the method is called Frequency Division Multiple Access (FDMA).
Similarly, instead on using different frequencies that are assigned to different users, signals can be transmitted at different time slots in a round-robin fashion, as shown on FIG. 2. Signals from two transceivers occupy the same frequency band but they are separated based on time of arrival. Multiple access is achieved in time domain and the method is called Time Division Multiple Access (TDMA).
One commonly used access method, shown in FIG. 3, is a combination of the previous two methods, which is known as Code Division Multiple Access (CDMA). Using the known Frequency Hopping (FH) CDMA, different users non-overlap in time and frequency domain to avoid interference.
Similar to FH-CDMA is the FH-TDMA access method that is shown in FIG. 4. System based on this access method has a smaller capacity than a FH-CDMA but it can be built in a more cost effective way by using a less complex transceiver at one link end. Example of such a FH-TDMA system can be found in current VTech 2.4 GHz and 5.8 GHz cordless telephones, which are based on World Digital Cordless Telephone (WDCT) standard. The WDCT standard was originally developed by Siemens, AG. The WDCT standard is a modification of Digital European Cordless Telephone (DECT) standard derived by adding frequency hopping in DECT TDMA based access. The WDCT standard is commonly used for cordless telephones in North America.
To avoid interference from other systems which have relatively narrow band transmissions, e.g., see FIG. 5, FDMA systems are utilizing a known method called Dynamic Channel Assignment (DCA) or Dynamic Channel Replacement (DCR). The idea is based upon detecting a presence of interference by measuring Receive Signal Strength Indication (RSSI), frame synchronization, and/or packet/bit loss errors. If RSSI, number of frame synchronization, and/or packet/bit loss errors are more than specified values, new replacement frequency RF channel are chosen by both link parties (See FIG. 6) to avoid narrow band transmission.
Effectively, same approach can be used for FH-TDMA based systems shown in FIG. 7 and FIG. 8. One specific implementation of this approach is disclosed in U.S. patent application Ser. No. 09/566,371, filed May 8, 2000, which is incorporated herein by reference in their entirety.
In the case of operation of two different FH-TDMA systems in the same band and in close proximity as shown in FIG. 9, generated interference is generally much harder to be avoided. The first problem is that the interfering FH-TDMA system tries to use the same whole-allocated band of the existing FH-TDMA system, therefore limiting the number of RF channels that are free of interference or that are ready to be used as replacement RF channels. The second problem is that the interfering FH-TDMA system transmits short period of time pseudo randomly on a given RF channel. Because of the presence of the interfering FH-TDMA system on a given RF channel is harder to detect, thresholds in terms of required RSSI levels, number of lost synchronization and/or packet/bit errors is harder to be saturated. After one period of time with interference will follow several periods without interference on a given RF channel.
One way of addressing this issue is to include the commercially known enhanced mode as implemented in some existing 2.4 GHz and 5.8 GHz systems. In the enhanced mode, essentially same speech data from one user connection is transmitted twice over two different RF channels at two different times. This is schematically shown in FIG. 10. The negative side of this method is that the capacity of the system in terms of number of users that can simultaneously use the system is reduced in the presence of FH-TDMA type of interfere.